Hi!
I've been using my slack 12.1 installation again for few weeks after messing my current up a bit. It definitely feels cozier to me than kde4. I tried to pinpoint whats there in kde3 and not yet in kde4 that makes me feel so.

I'm using customized kde3 with compiz while i used kde4 with kwin compositing in current.

Here it comes:
Dolphin splits only to two views - not enough for me, i switched to konqueror as default file manager in kde4. Also i'm using kio-slaves a lot.
Annoying file selection routine in dolphin's treeview in 'single mouse click' mode. In kde3 clicking on a row selects adjacent file.
I am most often using a konqueror's filterbar, but it takes too much space in dolphin to leave it always visible so i modified konqueror's toolbar to use that instead. But kde4's filterbar is case sensitive.
Kget crashes often when used as download manager for konqueror - annoying.
Undo does not work always right for 'send to trash' from konqueror, at the same time i like konqueror's feature to undo 'closed tabs'.
Audio previews in a file manager on mouseover are not possible yet with kde 4.2.4.
Ark's interface is wasting screen real estate a lot. It does not let me to compress to tar.bz2 from a kde service menus.
Cannot configure middle click in desktop to bring up (classic)k-menu.
Cannot configure middle click on a taskbar entry to quit the program.
Cannot disable all annoying(to me) plasma tooltip popups - k-menu, pager etc.
Kwin compositing flickers(a single white flash) during switch to dashboard view or from kde desktop to a desktop with fullscreen virtualbox on it - this white flash is irritating.
WinXP feels generally faster on this 12.1 slack than on slack64-current with the same version of virtualbox, both on the same computer.

So, these little things came up after returning to kde3 over few months of using kde 4.2.4. I still plan to switch to kde4 again, but probably will keep a partition with kde3 version of slack available also.

EDIT: few more:
Konqueror's bookmark editor crashes when organizing bookmarks.
Plasma forgets desktop icon positions sometimes after a crash.
Juk misbehaves and is unusable for listening very short audio clips because of crossfading, amarok is too fullfeatured for my use.
Everything else in kde4 not mentioned here is cool.

Add to list:
kdelirc (its in 4.3 but not 4.2 which ships with Slack current/13)

Also, krdc is crashy...

Are these Slackware specific settings? Perhaps you should get involved over at the kde project and help make your suggestions a reality.

One more: Cannot paste clipboard text into a file in konqueror in kde 4.2.4. An emty text file have to be created first and opened in text editor.

Of course these are not slackware specific missing features, but these little things are really quite hard to spot while they make an important part of an user experience, so i'm just trying to list them as i encounter any.

I have been submitting to kde4 bug list about the flicker issue along with one feature request. But i probably won't submit any other till i haven't tried kde4.3 yet.